1. Field of the Invention
This patent specification relates to the processing of seismic data and, more particularly, to processing of combined surface-recorded vibratory seismic data and borehole-recorded vibratory seismic data.
2. Background of the Invention
Surface-seismic waveform data are used to generate images of the sub-surface geological structures. In principle, the acoustic impedance at a specific location is calculated as the ratio between in-coming and the reflected energy. Vertical Seismic Profiling is a technique whereby reflection from the subsurface are recorded on down-hole geophones from wave fields generated by a surface source.
Most processing of seismic data is based on the a priori knowledge of the signature of the seismic wave generated by the source. In recent years, significant progress has been made in the use of seismic interferometry to enable subsurface imaging with signals previously considered as noise. This is typically achieved by a process in which new seismic responses corresponding to “virtual sources or receivers” are constructed by cross-correlating seismic observations made on a closed surface surrounding the object investigated. As is commonly done for most imaging based on seismic data, approximations allow use of incomplete spatial coverage. For examples, see: Schuster, G. T., 2001, Theory of Daylight/Interferometric Imaging-Tutorial: 63rd Meeting, EAGE, Expanded Abstracts, Session: A-32; Wapenaar, K., Dragonov, D., Thorbecke, J., and Fokkema, J., 2002, Theory of acoustic daylight imaging revisited: 72nd Ann. Internat. Mtg., Soc. Expl. Geophys., Extended Abstracts, 2269-2272; and Curtis, A., Gerstoft, P., Sato, H., Snieder, R., and Wapenaar, K., 2006, Seismic interferometry, turning noise into signal: The Leading Edge, 25, no. 9, 1082-1092.
More recently, Brandsberg-Dahl, S., Xiao, X., and Hornby, B., 2007, Surface Seismic Imaging with VSP Green's Functions, paper H012, 69th Mtg.: Eur. Assn. Geosci. Eng., London, 2007 (hereinafter referred to as “Brandsberg-Dahl, et al.”), proposes getting closer to a self-contained, data-driven process for turning acoustic data into images. The authors suggest correlating Walk-away VSP data into surface-seismic data provided the VSP and surface-seismic data are acquired using a similar aperture. With the borehole seismic data acquired all the way to the surface, this gives a migration that is independent of explicit determination of velocities. However the correlation-based technique may not generate satisfactory resolution in many applications.
One commonly used source with extended time signature is the Seismic Vibrator, a low-impact, low-amplitude, typically hydraulic, source programmed to generate a signal within a user-defined frequency range. The source signature has a duration of typically around 10-20 s, during which time the frequency of the driving signal continuously changes from a minimum value, typically 8 Hz, to a maximum value, typically around 100 Hz. One characteristic of a Seismic Vibrator is that its response is non-linear, such that the vibrator, in addition to the acoustic signal at the intended frequency, generates energy at higher harmonics at two, three, or even higher multiples of this frequency. A problem arises when the recordings of the surface and down-hole receivers are correlated with a reference signal not describing or improperly describing the harmonic energy, the harmonic energy representing an out-of-time occurrence of the higher frequencies will be misplaced, resulting in data containing not-properly-accounted-for energy at higher frequencies at nearly all times, eventually limiting the useful bandwidth of the data.